Navigation systems assist users in precisely locating objects. For instance, navigation systems are used in industrial, aerospace, and medical applications. In the medical field, navigation systems assist surgeons in precisely placing surgical instruments relative to a target site in a patient. The target site usually requires some form of treatment, such as tissue removal. In some cases, the target site is defined in the navigation system using a virtual object, such as a 3-D model. A representation of the virtual object can be displayed to the user during surgery to assist the user in visualizing placement of a treatment end of the instrument relative to the target site. For instance, the target site may be associated with a bone of the patient and the virtual object may define a volume of the bone to be removed by the treatment end of the instrument.
Conventional navigation systems employ a localizer that cooperates with trackers to provide position and/or orientation data associated with the instrument and the target site, e.g., the volume of the bone to be removed. The localizer is usually placed so that it has a field of view of the trackers. The trackers are fixed to the instrument and to the patient to move in concert with the instrument and the patient. The tracker attached to the patient is attached to the bone being treated thereby maintaining a rigid relationship with respect to the target site owing to the rigid nature of the bone. By using separate trackers on the instrument and the patient, the treatment end of the instrument can be precisely positioned to stay within the target site.
Often, the target site is located adjacent to sensitive anatomical structures, such as soft tissue, that should be avoided during surgery. These sensitive anatomical structures are difficult to track using conventional trackers, as these sensitive anatomical structures can shift relative to the trackers due to their elastic and/or flexible nature. Just as often, retractors or other tools are located near the target site that should also be avoided during the surgery. The retractors or other tools could be tracked in the same manner as the instrument being used for treating the patient, but adding trackers to the retractors and other tools can substantially increase costs and complexity in the navigation system, particularly by increasing the number of objects to be tracked by the navigation system. As a result, in current surgical procedures, avoidance is sometimes the responsibility of the user, so extreme care must be taken by the user to avoid sensitive anatomical structures and untracked tools that may be near the target site.
Thus, there is a need in the art for navigation systems and methods that address the identification of sensitive anatomical structures and/or other structures that are to be avoided during surgery.